Lost Planet: Extreme Condition walkthrough and gameplay questions

hello,
please tell me, after passing several levels, where then you
find yourself in a large cave, there is still a whole horde of flying pterodactyls, well, in general, after this level the troubles with shooting will start: press the mouse button / release, but the machine does not stop and fires the entire horn, before reloading and so it happens later on.
can someone tell me: why so?